This invention relates to the manufacture of integrated circuits, and in particular to slurries used for Chemical-Mechanical-Polishing of metal structures used in copper metallization.
As integrated circuit devices shrink, with semiconductor device geometries approaching 0.18 micron minimum feature size, and as circuit speed and performance increase, copper has replaced aluminum as the preferred electrical interconnect material. The use of copper as an interconnect material in silicon integrated circuits has occurred in response to the need for lowered interconnect resistivity, good electromigration resistance, and good deposition characteristics which allow effective filling of vias and contacts.
Copper metallization structures are often formed by a process known as Damascene, which is illustrated in FIG. 1. An insulating layer known as the Interlevel Dielectric (ILD) separates metal layers in a multilevel metallization structure. ILD dielectric layer 2, which may be comprised of a bottom layer 4 and a top, low dielectric constant layer 6, has regions 8 etched therein into which the metal lines will be inlaid. A barrier layer 10 is deposited, which serves to prevent diffusion of copper from the metal lines into the dielectric. This barrier layer is generally comprised of Ta or Ta compounds. A copper seed layer is then generally deposited, followed by an electroplated copper layer 14. The excess copper is then removed by a process known as Chemical Mechanical Polishing (CMP). CMP enhances the removal of surface material over large distances and short distances by simultaneously abrading the surface while a chemical etchant selectively attacks the surface. For this purpose, CMP utilizes a polishing slurry containing both an abrasive and a chemically active component.
Typically, in copper Damascene processing, the CMP is performed in two steps. The first CMP step removes the excess copper from the wafer surface, and may also remove part or all of the underlying barrier layer 10. A second CMP step is then generally performed, with the objectives of 1) completely removing the conductive Ta layer from the dielectric surface between Cu lines, and 2) planarizing the surface to compensate for Cu dishing and erosion, illustrated in FIG. 2. To accomplish the second objective, the second CMP step must have a selectively higher polish rate of SiO2 than of Cu, thereby compensating for Cu dishing during over-polish.
Of equal importance to these structural objectives is the quality of the polished surfaces, both Cu and SiO2, with respect to both surface damage/roughness and foreign materials on the surface. Post CMP cleaning can only address removable solid materials and ionic contamination.
The preferred abrasive used in slurries for Ta barrier polishing is silica, although other abrasives such as alumina have been used. The advantages to using silica abrasive in place of the alumina abrasive commonly used in other CMP applications include: 1) increased Ta removal rate, 2) greater ability to polish the oxide dielectric film for planarization, and 3) the potential for minimizing damage to the oxide and Cu surfaces. All of these advantages result from the high chemical reactivity of silica, resulting in a higher ratio of chemical to mechanical component of the polish than would occur using alumina abrasive. The hydrolysis of Sixe2x80x94Oxe2x80x94Si bonds to Sixe2x80x94OH HOxe2x80x94Si, and the reverse chemical process, namely, condensation of Sixe2x80x94OH HOxe2x80x94Si to Sixe2x80x94Oxe2x80x94Si+H2O, form the basis of much of the well documented chemistry of silica, as described by R. K. Iler in The Chemistry of Silica, Wiley-Interscience, New York, 1979. However, this high chemical reactivity poses difficult challenges in preventing unwanted reactions involving silica from occurring on the wafer surface.
A typical silica abrasive slurry used for Ta barrier polishing comprises 50-300 nm diameter silica particles suspended in an aqueous medium. To avoid the problem of copper corrosion during and after polish, copper corrosion inhibiting compounds such as benzotriazole or 1,2,4-triazole, are typically dissolved in the slurry medium, and the pH of the suspension is adjusted to a value between pH7 and pH10.5, which is the range empirically found to produce the lowest corrosion rates. Byproducts of the polishing process result in the slurry medium containing dissolved silica, dissolved copper, and dissolved tantalum, in addition to the formulating slurry ingredients.
In the prior art, two types of solid defects have been seen after CMP of copper features using silica slurries, and also after CMP of copper features using alumina slurries when SiO2 was present. These defects include precipitates and copper stains. The use of corrosion inhibiting triazole compounds in the slurry has been found to greatly amplify the occurrence of these defects. The precipitated residues, which are comprised in part of conducting materials, adversely affect device yield and reliability, for example by causing shorting and/or line-to-line leakage. Residues and precipitates additionally prevent the dielectric barrier from effectively sealing the top surface of the copper line, resulting in copper diffusion into the dielectric as well as providing a surface electromigration path for copper atoms.
It is therefore an object of this invention to provide an improved CMP slurry for the polishing of Ta barrier layers in copper metallization during integrated circuit processing which yields a lowered incidence of silica precipitates and copper stains.
It is a further object of this invention to provide a CMP slurry for the polishing of Ta barrier layers in copper metallization during integrated circuit processing which includes corrosion-inhibiting triazole compounds, which further includes silica abrasive, and which yields a lowered incidence of silica precipitates and copper stains.
It is a further object of this invention to provide a CMP slurry for the polishing of Ta barrier layers in copper metallization during integrated circuit processing which inhibits chemical reactions between silica, triazole, and copper.
Our invention meets these objects by providing a CMP slurry for the polishing of Ta barrier layers underlying copper metallization which includes at least one additional slurry component which inhibits silica-triazole-copper reactions. A set of chemical compounds has been successfully used in a CMP slurry to inhibit said reactions, including organic compounds which form hydrogen bonds to the surface of polymeric silica molecules with a high degree of surface coverage, and which also adsorb onto copper hydroxo species.